when you load a partially linked object file, or when a function you
compiled calls a C function that is not defined in any previously loaded
library or module you get the above error. To find out which functions
are not defined, type (mod-undefined)
.

11.0.1. compiler : Unknown Type in: ()

This message may occur for a variety of reasons, but the most common one
is that the compiler can't figure out the type of the return value of an
expression. A frequent cause is an hash-brace construct (inline C code
segment) that is the last expression of a function (and therefore its
return value). Lush cannot know the type of the value returned by a
hash-brace unless you cast it.

Some functions in popular libraries take function pointers as argument
(a typical example is the GSL function minimization routines). To obtain
a pointer to the compiled version is a function written in Lisp, simply
use the function to-gptr :